A number of approaches have been used for the bulk separation of carbon dioxide from methane. Conventional technology in the natural gas industry uses amine, a liquid absorbent, to remove CO.sub.2, but these plants are generally suitable only for very large volumes of gas, and will not work in the presence of oxygen, such as typically found in landfill gas and coal mine gob gas. Another liquid absorbent, a glycol derivative with the trade name Selexol has been successfully used with landfill gas, but it requires pressurization up to 300 psi and is fairly expensive.
For smaller volume applications where gas flows are less than five or ten million cubic feet per day, considerable attention has been given to the development of pressure swing adsorption (PSA) processes. This technology is based on the tendency of solids to attract or bind gaseous molecules to their surface, and for some solids to attract certain gases more strongly than other gases. Typically, the higher the pressure of the gas, the more is adsorbed on the surface of the solid. A curve representing this relationship at a constant temperature (temperature also has an effect on the amount of gas adsorbed) is called an isotherm. Though adsorption is not well understood, it is thought that the adsorbed gas forms a partial or complete layer only one or at most a few molecules thick on the surface of the solid. This layer can be thought of as being a liquid state of the gas. Sometimes a gas is not desorbed as readily as it is adsorbed, so that the desorption isotherm will show hysteresis in comparison to the adsorption isotherm. Gas separations using pressure swing adsorption processes are based on the selective adsorption on the solid, or adsorbent, of some gases over others as pressures are increased, thereby concentrating the gas that is less strongly adsorbed. When the pressure is decreased the adsorbed gas is desorbed, thereby regenerating the solid for successive cycles of adsorption and desorption. Solids used in PSA processes are typically those that have very large surface areas, such as activated carbon, silica gel, or molecular sieves (zeolites), which have the added advantage of being able to screen out gases having molecular diameters larger than the pores in the zeolite.
Numerous patents describe PSA processes for separating carbon dioxide from methane or other gases. They have in common the use of one of the adsorbents described above that adsorb carbon dioxide preferentially over methane. A typical design has two or more adsorbent columns that go through a series of cycles involving pressurization, flow of a primary product gas containing predominantly methane, depressurization and regeneration with a secondary product containing predominantly carbon dioxide. There are many variations in handling the depressurization or blowdown gas in efforts to conserve the energy of pressurization and minimize methane losses. Also, there are many approaches to purging and re-pressurizing the columns using primary and secondary product gases. Concurrent or counter current flows are used in an effort to increase the purity of the product(s). Many PSA processes may require high operating pressures, are not successful in achieving a good separation, or use expensive adsorbents. The lack of an economical process for removing carbon dioxide from landfill gas is believed to be the main reason very few landfill operations attempt to make a high BTU fuel.
One of the earlier patents in this area is U.S. Pat. No. 3,751,878, which describes a PSA system using a zeolite molecular sieve that selectively adsorbs CO.sub.2 from a low quality natural gas stream operating at a pressure of 1000 psia, and a temperature of 300.degree. F. The system uses carbon dioxide as a purge to remove some adsorbed methane from the zeolite and to purge methane from the void space in the column. U.S. Pat. No. 4,077,779, describes the use of a carbon molecular sieve that adsorbs CO.sub.2 selectively over hydrogen or methane. After the adsorption step, a high pressure purge with CO.sub.2 is followed by pressure reduction and desorption of CO.sub.2 followed by a rinse at an intermediate pressure with an extraneous gas such as air. The column is then subjected to vacuum to remove the extraneous gas and any remaining CO.sub.2.
U.S. Pat. No. 4,770,676, describes a process combining a temperature swing adsorption (TSA) process with a PSA process for the recovery of methane from landfill gas. The TSA process removes water and minor impurities from the gas, which then goes to the PSA system, which is similar to that described in U.S. Pat. No. 4,077,779 above, except the external rinse step has been eliminated. CO.sub.2 from the PSA section is heated and used to regenerate the TSA section. U.S. Pat. No. 4,857,083, claims an improvement over U.S. Pat. No. 4,077,779 by eliminating the external rinse step and using an internal rinse of secondary product gas (CO.sub.2) during blowdown, and adding a vacuum for regeneration. The preferred type of adsorbent is activated carbon, but can be a zeolite such as 5A, molecular sieve carbons, silica gel, activated alumina or other adsorbents selective of carbon dioxide and gaseous hydrocarbons other than methane.
U.S. Pat. No. 4,915,711, describes a PSA process that uses adsorbents from essentially the same list as above, and produces two high purity products by flushing the product (methane) from the column with the secondary product (carbon dioxide) at low pressure, and regenerating the adsorbent using a vacuum of approximately 1 to 4 psia. The process includes an optional step of pressure equalization between columns during blowdown. U.S. Pat. No. 5,026,406 is a continuation in part of U.S. Pat. No. 4,915,711 with minor modifications of the process.
Use of the zeolite clinoptilolite has been described in three relevant patents. U.S. Pat. No. 4,935,580, describes a process for removing trace amounts of water or CO.sub.2 from a hydrocarbon gas stream using an ion-exchanged clinoptilolite. U.S. Pat. No. 5,587,003, describes using natural or synthetic clinoptilolite as an adsorbent to remove carbon dioxide at very low partial pressures, i.e. in trace amounts, from a gas stream, particularly air. U.S. Pat. No. 4,964,889, describes a process for using a clinoptilolite that has been ion-exchanged to have a high proportion of magnesium ions to separate nitrogen from methane in a PSA process. Included in the description is the necessity to remove water from the clinoptilolite by activation at temperatures of 350.degree. to 700.degree. C.